Magnetic nanoparticles of zinc/calcium ferrite decorated with silver for photodegradation of dyes

materials Article Magnetic Nanoparticles of Zinc/Calcium Ferrite Decorated with Silver for Photodegradation of Dyes Ricardo J. C. Fernandes 1, Carlos A. B. Magalhães 1, Carlos O. Amorim 2, Vítor S. Amaral 2, Bernardo G. Almeida 1, Elisabete M. S. Castanheira 1and Paulo J. G. Coutinho 1,* 1Centre of Physics (CFUM), University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal 2Physics Department and CICECO, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal *Correspondence: [email protected] Received: 2 October 2019; Accepted: 29 October 2019; Published: 31 October 2019   Abstract: Magnetic nanoparticles of zinc/calcium ferrite and decorated with silver were prepared by coprecipitation method. The obtained nanoparticles were characterized by UV/Visible absorption, XRD, TEM and SQUID. The mixed zinc/calcium ferrites exhibit an optical band gap of 1.78 eV. HR-TEM imaging showed rectangular nanoplate shapes with sizes of 10 ± 3 nm and aspect ratio mainly between 1 and 1.5. Magnetic measurements indicated a superparamagnetic behavior. XRD diffractograms allowed a size estimation of 4 nm, which was associated with the nanoplate thickness. The silver-decorated zinc/calcium ferrite nanoparticles were successfully employed in the photodegradation of a model dye (Rhodamine B) and industrial textile dyes (CI Reactive Red 195, CI Reactive Blue 250 and CI Reactive Yellow 145). The nanosystems developed exhibited promising results for industrial application in effluent photoremediation using visible light, with the possibility of magnetic recovery. Keywords: magnetic nanoparticles; zinc/calcium ferrite; silver decorated nanoparticles; photodegradation; textile dyes 1. Introduction Nowadays, one of the major problems worldwide is water pollution. Considering population growth and the consequent increase in industrialization, pollution levels in water resources have grown dramatically. The textile industry appears as one of the most polluting sectors worldwide. It deals daily with millions of liters of water, leaving an associated trace of color in its effluents, which represents one of the main problems of this industrial sector [1,2]. Recently, several works have drawn attention to nanotechnology for environmental applications and specifically to magnetic nanoparticles, some of them being capable of degrade textile dyes by photodegradation [3]. For many years, titanium dioxide has been used as the photocatalyst of excellence. However, its large band gap of 3.2 eV decreases its applicability, by only degrading in the presence of UV light [ 4 ]. Lower energy radiation (e.g. in the visible spectrum) can be used if the bandgap of the semiconductor is reduced. In this context, zinc ferrites appear as a promising semiconductor (band gap of 1.9 eV), promoting photodegradation of dyes under visible light, as demonstrated in recent studies [ 4 ]. However, focusing on photoremediation of industrial effluents, the magnetic properties of the nanoparticles must be improved, to allow magnetic recovery and reuse of the photocatalysts. Biocompatibility is also a feature to pursue, considering applications in the photoremediation of water natural resources. The incorporation of calcium in the nanoparticles composition, giving mixed zinc/calcium ferrite nanoparticles, allows obtaining stable ferrites with enhanced biocompatibility and magnetic properties [5,6]. Materials 2019,12, 3582; doi:10.3390/ma12213582 www.mdpi.com/journal/materials Materials 2019,12, 3582 2 of 16 One of the main limitations for the application of ferrite nanoparticles is their low separation efficiency of electrons and holes, which leads to a much lower photocatalytic activity comparing to titanium dioxide [ 4 ]. It has been shown that the deposition of a noble metal at nanoparticle surface increases the separation rate of electrons and holes, promoting the transfer to the interfacial load [ 7 , 8 ]. In the present work, silver was used to cover the nanoparticles surface [ 4 , 7 , 8 ]. Since the rate of recombination of ferrites is high, reducing their photocatalytic activity, the incorporation of silver reduces rapid recombination of the generated electron/hole pairs, increasing the formation of reactive species and allowing an enhanced photocatalytic activity. In this work, mixed zinc/calcium ferrite nanoparticles decorated with silver clusters were tested as photodegradation agents for textile reactive azo dyes, namely Reactive Red 195 (“Red”), Reactive Blue 250 (“Blue”) and Reactive Yellow 145 (“Yellow”). These dyes have a general structure R − N= N−R ’ (Table 1) and are the most used class in industrial dyeing processes, being generally persistent in final industrial effluents [9,10]. Rhodamine B (structure in Table 1) was also used as model dye for comparison, due to its well-known photophysical properties [ 11 , 12 ] and wide use in photodegradation assays [13–15]. The proposed nanoparticles are advantageous for waste water treatment, as the incorporation of Zn cations may promote antimicrobial activity and the presence of calcium in the ferrite structure favors biocompatibility of the nanoparticles. Studies in cell lines have shown that, for a 100 µ g/mL concentration of nanoparticles, zinc ferrite allows 70.5% of cell viability at 24 h, while calcium ferrite allows 90.6% of cell viability for the same concentration and time of exposure. For comparison, using cobalt ferrite nanoparticles, 80% of cell viability was observed but with only 20 µ g/mL of nanoparticles [ 16 ]. Therefore, enhanced biocompatibility is expected by inclusion of calcium in zinc ferrites. The nanosystems here developed show promising results for industrial application in effluent photoremediation. Materials 2019,12, 3582 3 of 16 Table 1. Structure of the textile dyes and model dye used for photodegradation assays. Commercial Name Molecular Formula Molecular Weight (g/moL) Molecular Structure C.I. Reactive Blue 250 (Reactive Blue RGB) C27H23N5Na4O20S61021.84 Materials 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/materials Table 1. Structure of the textile dyes and model dye used for photodegradation assays. Commercial Name Molecular Formula Molecular weight (g/mol) Molecular structure C.I. Reactive Blue 250 (Reactive Blue RGB) C 27 H 23 N 5 Na 4 O 20 S 6 1021.84 C.I. Reactive Yellow 145 (Reactive Yellow 3RS) C 28 H 20 ClN 9 Na 4 O 16 S 5 1026.25 C.I. Reactive Red 195 (Reactive Red 3BS) C 31 H 19 ClN 7 Na 5 O 19 S 6 1136.32 C.I. Reactive Yellow 145 (Reactive Yellow 3RS) C28H20ClN9Na4O16S51026.25 Materials 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/materials Table 1. Structure of the textile dyes and model dye used for photodegradation assays. Commercial Name Molecular Formula Molecular weight (g/mol) Molecular structure C.I. Reactive Blue 250 (Reactive Blue RGB) C 27 H 23 N 5 Na 4 O 20 S 6 1021.84 C.I. Reactive Yellow 145 (Reactive Yellow 3RS) C 28 H 20 ClN 9 Na 4 O 16 S 5 1026.25 C.I. Reactive Red 195 (Reactive Red 3BS) C 31 H 19 ClN 7 Na 5 O 19 S 6 1136.32 C.I. Reactive Red 195 (Reactive Red 3BS) C31H19ClN7Na5O19S61136.32 Materials 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/materials Table 1. Structure of the textile dyes and model dye used for photodegradation assays. Commercial Name Molecular Formula Molecular weight (g/mol) Molecular structure C.I. Reactive Blue 250 (Reactive Blue RGB) C 27 H 23 N 5 Na 4 O 20 S 6 1021.84 C.I. Reactive Yellow 145 (Reactive Yellow 3RS) C 28 H 20 ClN 9 Na 4 O 16 S 5 1026.25 C.I. Reactive Red 195 (Reactive Red 3BS) C 31 H 19 ClN 7 Na 5 O 19 S 6 1136.32 Rhodamine B C28H31ClN2O3479.02 Materials 2019, 12, x FOR PEER REVIEW 2 of 18 Rhodamine B C 28 H 31 ClN 2 O 3 479.02 Materials 2019,12, 3582 4 of 16 2. Materials and Methods 2.1. Nanoparticles Preparation 2.1.1. Zinc/Calcium Ferrite Nanoparticles Zinc/calcium ferrite nanoparticles were prepared through a coprecipitation method in reflux conditions, adapting a previously described procedure by Cao et al. [ 4 ]. First, 1.082 g of iron (III) chloride hexahydrate, 0.219 g of zinc acetate and 0.158 g of calcium acetate were dissolved in 200 mL of ultrapure water Milli-Q grade (MilliporeSigma, St. Louis, MO, USA). After dissolution, 1 mL of oleic acid and 1.198 g of urea were added to the solution. After complete dispersion, the solution was refluxed vigorously for at least 3 h. For purification, the obtained sample was washed several times with absolute ethanol and ultrapure water, by magnetic decantation and centrifugation (14,000 g). The mixed ferrite nanoparticles were dried for 12 h at 90 ◦ C. To improve crystallinity, the zinc/calcium ferrite nanoparticles were calcined at 400 ◦C for 30 min. 2.1.2. Zinc/Calcium Ferrites Decorated with Silver Clusters The as-prepared mixed ferrite nanoparticles (either calcined or non-calcined) were dispersed in 100 mL of ethylene glycol. Next, 0.160 g of silver nitrate were dissolved in 20 mL of ultrapure water and added to the previous dispersion. This solution was refluxed for 30 min. The products were separated by magnetic decantation and centrifugation (14,000 g) and washed repeatedly with absolute ethanol. The nanoparticles were dried for 12 h at 90 ◦C. 2.2. Structural Characterization 2.2.1. Transmission Electron Microscopy (TEM) TEM images of nanoparticles were acquired using a Transmission Electron Microscope JEOL 2100 (JEOL USA Inc., Peabody, MA, USA) operating at 200 kV coupled to an Electron Dispersive X-Ray Spectroscopic analyzer (EDS). The solutions were sonicated in ethanol and dropped onto a TEM grid (copper 400 mesh with a carbon film). TEM images were processed using ImageJ 1.52p software (National Institutes of Health (NIH), Bethesda, MD, USA). The size of each particle was determined by equalizing its area with the area of a circle. However, this is a crude approximation for the type of particles observed in TEM images. Thus, an additional estimation was made by inscribing rectangular shapes on each particle. The ratio of resulting side lengths of the obtained rectangles was used as an estimation of the aspect ratio. 2.2.2. X-Ray Diffraction (XRD) X-Ray Diffraction (XRD) analyses were performed using a conventional Philips PW 1710 (Royal Philips, Amsterdam, The Netherlands) diffractometer, operating with CuK α radiation, in a Bragg-Brentano configuration. 2.2.3. Magnetic Measurements Magnetization measurements were done in a MPMS3 SQUID magnetometer (Quantum Design Inc., San Diego, CA, USA). The hysteresis cycles (magnetization versus magnetic field) of the samples were measured in the convenient field range for each sample, with a possible maximum +/ − 70 kOe (+/ − 7 Tesla). The measurement method was by DC extraction or VSM oscillation at a frequency of 14 Hz. A specific magnetic field correction for the trapped flux in the superconducting coil was made achieving an accuracy of residual less than 2 Oe. Materials 2019,12, 3582 5 of 16 2.3. Photodegradation Assays To evaluate the photocatalytic activity of the as-prepared nanoparticles, a home-built irradiation apparatus was used. The setup incorporates a 200 W Xenon Arc Lamp (L.O.T.-Oriel GmbH & Co. KG, Darmstadt, Germany), a 400 nm long pass filter (Thorlabs Inc., Newton, NJ, USA) to isolate the visible spectrum radiation, and a sample cuvette holder. Aqueous solutions of Rhodamine B (40 mg/L) and of textile dyes, C. I. Reactive Red 195 (“Red”), C. I. Reactive Blue 250 (“Blue”) and C. I. Reactive Yellow 145 (“Yellow”) (80 mg/L) were assayed for 2.5 h. In the first 30 min, the nanoparticles were added to the solution, in constant stirring, under dark. After this initial time, the sample solution was exposed to light under magnetic stirring, and aliquots were taken at 0, 5, 10, 15, 30, 60, 90 and 120 min. The photocatalyst content of each aliquot was removed by centrifugation and the absorption spectra were recorded in a Shimadzu UV-3600 Plus UV-Vis-NIR (Shimadzu Corporation, Kyoto, Japan) spectrophotometer. 3. Results and Discussion 3.1. Nanoparticles Characterization 3.1.1. Absorption Spectra Figure 1displays the UV-Visible absorption spectra of aqueous dispersions of mixed zinc/calcium ferrite nanoparticles and Ag-decorated zinc/calcium ferrite nanoparticles. Materials 2019, 12, x FOR PEER REVIEW 2 of 17 2.2.3. Magnetic Measurements Magnetization measurements were done in a MPMS3 SQUID magnetometer (Quantum Design Inc., San Diego, CA, USA). The hysteresis cycles (magnetization versus magnetic field) of the samples were measured in the convenient field range for each sample, with a possible maximum +/−70 kOe (+/−7 Tesla). The measurement method was by DC extraction or VSM oscillation at a frequency of 14 Hz. A specific magnetic field correction for the trapped flux in the superconducting coil was made achieving an accuracy of residual less than 2 Oe. 2.3. Photodegradation Assays To evaluate the photocatalytic activity of the as-prepared nanoparticles, a home-built irradiation apparatus was used. The setup incorporates a 200 W Xenon Arc Lamp (L.O.T.-Oriel GmbH & Co. KG, Darmstadt, Germany), a 400 nm long pass filter (Thorlabs Inc., Newton, NJ, USA) to isolate the visible spectrum radiation, and a sample cuvette holder. Aqueous solutions of Rhodamine B (40 mg/L) and of textile dyes, C. I. Reactive Red 195 (“Red”), C. I. Reactive Blue 250 (“Blue”) and C. I. Reactive Yellow 145 (“Yellow”) (80 mg/L) were assayed for 2.5 hours. In the first 30 minutes, the nanoparticles were added to the solution, in constant stirring, under dark. After this initial time, the sample solution was exposed to light under magnetic stirring, and aliquots were taken at 0, 5, 10, 15, 30, 60, 90 and 120 minutes. The photocatalyst content of each aliquot was removed by centrifugation and the absorption spectra were recorded in a Shimadzu UV-3600 Plus UV-Vis-NIR (Shimadzu Corporation, Kyoto, Japan) spectrophotometer. 3. Results and Discussion 3.1. Nanoparticles Characterization 3.1.1. Absorption Spectra Figure 1 displays the UV-Visible absorption spectra of aqueous dispersions of mixed zinc/calcium ferrite nanoparticles and Ag-decorated zinc/calcium ferrite nanoparticles. Figure 1. UV-Visible absorption spectra of aqueous dispersions of (a) zinc/calcium ferrite nanoparticles and (b) silver-decorated nanoparticles. The spectrum of the mixed zinc/calcium ferrite nanoparticles in Figure 1a allows the determination of the optical band gap, using a Tauc plot (Equation (1)), () ( ) g nEhναhν−∝ (1) where α is the absorption coefficient (proportional to the absorbance), n is an exponent that depends on the nature of the transition (being n = 2 for a direct semiconductor and n = 1/2 for an indirect one) and Eg is the optical band gap [17]. A band gap of 1.78 eV (n = 2) was estimated from the intercept of inset of Figure 1a, in agreement with the value of 1.90 eV reported by Kim et al. for calcium ferrite nanoparticles [18], as well as for zinc ferrite [4]. Figure 1. UV-Visible absorption spectra of aqueous dispersions of ( a ) zinc/calcium ferrite nanoparticles and (b) silver-decorated nanoparticles. The spectrum of the mixed zinc/calcium ferrite nanoparticles in Figure 1a allows the determination of the optical band gap, using a Tauc plot (Equation (1)), (αhν)n∝hν−Eg(1) where α is the absorption coefficient (proportional to the absorbance), n is an exponent that depends on the nature of the transition (being n =2 for a direct semiconductor and n =1/2 for an indirect one) and E g is the optical band gap [ 17 ]. A band gap of 1.78 eV (n =2) was estimated from the intercept of inset of Figure 1a, in agreement with the value of 1.90 eV reported by Kim et al. for calcium ferrite nanoparticles [18], as well as for zinc ferrite [4]. Comparingthenanoparticleswithoutandwithsilver(Figure1), itcanbeobservedthecharacteristic local surface plasmon resonance (LSPR) band of silver nanoparticles around 435 nm, within the range of values previously reported [19]. 3.1.2. X-Ray Diffraction (XRD) Measurements The calcination process allows an improvement in crystallinity and magnetic properties of the nanoparticles, which is essential for their recovery at the end of the irradiation procedure, enabling the Materials 2019,12, 3582 6 of 16 possibility to recycle and reuse the nanoparticles [ 20 , 21 ]. XRD analysis revealed a strongly amorphous background for the non-calcined nanoparticles (Figure 2a). Upon calcination, several well defined diffraction peaks are observed (Figure 2b). Using FullProf software (version 5.8, J. Rodr í guez-Carvajal, Lab. L é on Brillouin, Gif sur Yvette, France) [ 22 ], Rietveld analysis of calcined zinc/calcium ferrite diffractogram was performed, by adapting CIF file number 2300615 (partially inverted cubic spinel phase, space group Fd 3 m), corresponding to zinc ferrite, to have 50% occupation with Zn and 50% with Ca at the zinc lattice sites. Bulk zinc ferrite has a direct spinel structure. However, it was found that in nanoparticles the degree of inversion, i, increases with the decrease of nanoparticle size [ 23 ], with a corresponding enhancement of magnetic properties. Recently, it was reported that mixed zinc/calcium ferrites adopt an inverted spinel structure [ 24 ]. Thus, an inverted spinel structure is considered, in which the A 2+ ions in octahedral sites are 50% distributed between zinc and calcium: (Fe) Td (FeZn 0.5 Ca 0.5 ) Oh O 4 . A reasonable value of R F =4.35 (Table 2) was obtained, indicating that the assumed crystal structure is compatible with the XRD results, since all the corresponding diffraction peaks are observed and have nearly the calculated intensities (Figure 2b). Materials 2019, 12, x FOR PEER REVIEW 4 of 17 Figure 2. XRD diffractograms of zinc/calcium ferrite nanoparticles: (a) Non-calcined zinc/calcium ferrite; (b) calcined zinc/calcium ferrite; (c) non-calcined zinc/calcium ferrite decorated with silver; (d) calcined zinc/calcium ferrite decorated with silver. Gray lines: Experimental patterns; black lines: fitted patterns. Miller indices: Black: zinc/calcium ferrite; Red: Silver. Table 2. Selected Rietveld analysis parameters. Sample Ox,y,z (*) i (*) Phase size (nm) Lattice constant (nm) Zn/Ca ferrite|Ag Rf Zn/Ca ferrite|Ag X2 Zn/Ca ferrite non-calcined 0.2405 1 (+) 1.12 | ---- 0.8425 (+) | ---- 3.18 | ---- 1.11 Zn/Ca ferrite calcined 0.2464 1 (+) 3.97 | ---- 0.8425 | ---- 4.35 | ---- 1.26 Zn/Ca ferrite non-calcined with silver 0.2405 (+) 1 (+) 1.12 (+) | 3.41 0.8425 (+) | 0.4069 3.42 | 0.95 1.32 Zn/Ca ferrite calcined with silver 0.2464 (+) 1 (+) 3.97 (+) | 9.90 0.8425 (+) | 0.4078 9.81 | 2.91 1.13 (*) Values in CIF file 2300615 are Ox,y,z = 0.2535 and i = 0.62; (+) fixed value. It can be observed (Table 3) that the Ag coupled calcined nanoparticles exhibit a lower silver content, indicating that silver exhibits more affinity for the non-calcined amorphous nanoparticles. However, the significant enhancement of the crystalline structure of the nanoparticles with calcination is determinant in obtaining suitable magnetic properties for environmental applications. Table 3. Estimated percentage of silver in the nanoparticles obtained by XRD. Nanoparticles Zn0.5Ca0.5Fe2O4 (%) Ag (%) Zn0.5Ca0.5Fe2O4 non-calcined 100 - Zn0.5Ca0.5Fe2O4 calcined 100 - [email protected] non-calcined 57.8 42.2 [email protected] calcined 66.4 33.6 3.1.3. Transmission Electron Microscopy (TEM) Figure 2. XRD diffractograms of zinc/calcium ferrite nanoparticles: ( a ) Non-calcined zinc/calcium ferrite; ( b ) calcined zinc/calcium ferrite; ( c ) non-calcined zinc/calcium ferrite decorated with silver; ( d ) calcined zinc/calcium ferrite decorated with silver. Gray lines: Experimental patterns; black lines: fitted patterns. Miller indices: Black: zinc/calcium ferrite; Red: Silver. The average size that results from Debye-Scherrer equation, as implemented by FullProf suite [ 22 ], is 3.97 nm and the lattice constant is 8.425 Å. The lattice constant for bulk ZnFe 2 O 4 is 8.443 Å [ 25 ], but values down to 8.411 Å using thermal decomposition method [ 25 ], and 8.391 Å using microwave synthesis [ 26 ], and up to 8.47 Å when using coprecipitation method [ 23 ], were reported. Calcium ferrite nanoparticles in the spinel crystallographic form and using co-precipitation methods have lattice constants between 8.34 Å [ 27 ] and 8.37 Å [ 28 ]. Thus, the lattice constant of the here obtained zinc/calcium mixed ferrite lies between the corresponding single ferrite phases. Rietveld analysis on the non-calcined sample (Figure 2a) is compatible with 1.1 nm size. Both calcined and non-calcined samples were coupled with metallic silver. Its presence is confirmed in the corresponding XRD diffractograms presented in Figure 2c,d, respectively. The Rietveld analysis using an additional phase corresponding to silver (CIF 9008459) allows an estimation of size of the coupled silver nanoparticles, Materials 2019,12, 3582 7 of 16 as well as of their amount in each sample. A summary of the Rietveld analysis of all samples is shown in Table 2and the resulting weight percentages of ferrite and silver are indicated in Table 3. Table 2. Selected Rietveld analysis parameters. Sample Ox,y,z (*) i (*) Phase Size (nm) Lattice Constant (nm) Zn/Ca Ferrite|Ag Rf Zn/Ca Ferrite|Ag X2 Zn/Ca ferrite non-calcined 0.2405 1 (+)1.12 |---- 0.8425 (+)|---- 3.18 |---- 1.11 Zn/Ca ferrite calcined 0.2464 1 (+)3.97 |---- 0.8425 |---- 4.35 |---- 1.26 Zn/Ca ferrite non-calcined with silver 0.2405 (+) 1 (+)1.12 (+)|3.41 0.8425 (+)|0.4069 3.42 |0.95 1.32 Zn/Ca ferrite calcined with silver 0.2464 (+) 1 (+)3.97 (+)|9.90 0.8425 (+)|0.4078 9.81 |2.91 1.13 (*) Values in CIF file 2300615 are Ox,y,z =0.2535 and i =0.62; (+) fixed value. Table 3. Estimated percentage of silver in the nanoparticles obtained by XRD. Nanoparticles Zn0.5Ca0.5Fe2O4(%) Ag (%) Zn0.5Ca0.5Fe2O4non-calcined 100 - Zn0.5Ca0.5Fe2O4calcined 100 - Ag@Zn 0.5 Ca 0.5 Fe 2 O 4 non-calcined 57.8 42.2 [email protected] 66.4 33.6 It can be observed (Table 3) that the Ag coupled calcined nanoparticles exhibit a lower silver content, indicating that silver exhibits more affinity for the non-calcined amorphous nanoparticles. However, the significant enhancement of the crystalline structure of the nanoparticles with calcination is determinant in obtaining suitable magnetic properties for environmental applications. 3.1.3. Transmission Electron Microscopy (TEM) TEM images of the calcined zinc/calcium ferrite nanoparticles (Figure 3A,B) revealed generally rod-like or prismatic shapes, with a size distribution of 10 ± 3 nm (Figure 3C), obtained considering circles with the same area of each of the 206 particles that were manually delimited. Assuming instead a rectangular shape (ImageJ bonding rectangle), sizes of longer and shorter sides are, respectively, 12 ±3 nm and 9.8 ±3 nm, with a broad aspect ratio distribution between 1.04 and 2 (Figure 3D). The difference in size from XRD estimation might be related to a nanoplate-like structure, already reported for zinc ferrite [ 4 ], where its thickness corresponds to the size determined by XRD (the nanoplates are lying down, so that only their thickness contributes to the amount of lattice planes that define the X-ray diffraction signal). EDX analysis (average of 5 measurements) allowed obtaining a ratio of Zn/Fe atomic percentages of 26.7%, in accordance to what was expected for Zn 0.5 Ca 0.5 Fe 2 O 4 (Zn/Fe ratio of 25%). TEM images of zinc/calcium ferrite nanoparticles decorated with silver clusters (Figure 3E,F) show the additional appearance of more spherical shapes (marked on Figure 3E,F) and also of agglomerates of these spherical particles. These have sizes of 9.4 ± 1 nm and correspond to the silver content of the prepared sample, being compatible with the size estimation obtained from XRD. EDX analysis estimated an atomic silver percentage of 21%, slightly smaller than the determined by XRD. Materials 2019,12, 3582 8 of 16 Materials 2019, 12, x FOR PEER REVIEW 5 of 17 TEM images of the calcined zinc/calcium ferrite nanoparticles (Figure 3A,B) revealed generally rod-like or prismatic shapes, with a size distribution of 10 ± 3 nm (Figure 3C), obtained considering circles with the same area of each of the 206 particles that were manually delimited. Assuming instead a rectangular shape (ImageJ bonding rectangle), sizes of longer and shorter sides are, respectively, 12 ± 3 nm and 9.8 ± 3 nm, with a broad aspect ratio distribution between 1.04 and 2 (Figure 3D). The difference in size from XRD estimation might be related to a nanoplate-like structure, already reported for zinc ferrite [4], where its thickness corresponds to the size determined by XRD (the nanoplates are lying down, so that only their thickness contributes to the amount of lattice planes that define the X-ray diffraction signal). EDX analysis (average of 5 measurements) allowed obtaining a ratio of Zn/Fe atomic percentages of 26.7%, in accordance to what was expected for Zn0.5Ca0.5Fe2O4 (Zn/Fe ratio of 25%). TEM images of zinc/calcium ferrite nanoparticles decorated with silver clusters (Figure 3E,F) show the additional appearance of more spherical shapes (marked on Figure 3E,F) and also of agglomerates of these spherical particles. These have sizes of 9.4 ± 1 nm and correspond to the silver content of the prepared sample, being compatible with the size estimation obtained from XRD. EDX analysis estimated an atomic silver percentage of 21%, slightly smaller than the determined by XRD. The position of the LSPR band (Figure 1B) depends on the size, shape and refractive index of the medium surrounding the silver nanoparticle. Considering silver nanospheres in water, for which citrate was used as stabilizing agent, the plasmon band for 10 nm size should appear at 398 nm [29]. However, in this case, the deposited silver particles are expected to be either nanodisks or half-spheres, with one side surrounded by the Zn/Ca mixed ferrite and the other side facing an aqueous environment. An increase in refractive index is expected to induce a red shift in the plasmon band. The refractive index of ZnFe2O4 in 400–500 nm region is above 2 [30], so that a significant red shift from 398 nm is expected. Also, the shape and its aspect ratio have a pronounced effect on the LSPR band position, with nanodisks of 10 nm diameter and 2 nm height showing two plasmon bands, one at ∼420 nm and the other, more intense, at ∼560 nm [19]. Thus, the observed plasmon band at 435 nm is not incompatible with the ∼10 nm size determined by TEM and XRD. E F C D B A Figure 3. TEM images of the synthesized nanoparticles. ( A , B ): Zinc/calcium ferrite nanoparticles; ( C ): Particle size histogram of image ( B ) and fitting to a Gaussian distribution; ( D ): Aspect ratio histogram of particles in image (B); (E,F): Zinc/calcium ferrite nanoparticles containing silver. The position of the LSPR band (Figure 1B) depends on the size, shape and refractive index of the medium surrounding the silver nanoparticle. Considering silver nanospheres in water, for which citrate was used as stabilizing agent, the plasmon band for 10 nm size should appear at 398 nm [ 29 ]. However, in this case, the deposited silver particles are expected to be either nanodisks or half-spheres, with one side surrounded by the Zn/Ca mixed ferrite and the other side facing an aqueous environment. An increase in refractive index is expected to induce a red shift in the plasmon band. The refractive index of ZnFe 2 O 4 in 400–500 nm region is above 2 [ 30 ], so that a significant red shift from 398 nm is expected. Also, the shape and its aspect ratio have a pronounced effect on the LSPR band position, with nanodisks of 10 nm diameter and 2 nm height showing two plasmon bands, one at ~420 nm and the other, more intense, at ~560 nm [ 19 ]. Thus, the observed plasmon band at 435 nm is not incompatible with the ~10 nm size determined by TEM and XRD. Small area electron diffraction (SAED) images of zinc/calcium ferrite samples without (Figure 4A) and with silver (Figure 4B) show diffraction spots that can be associated with ferrite phase (cyan rings) and silver (orange rings), as follows. The circular profile of the images was obtained using the radial profile ImageJ plugin and fitted to a sum of Gaussian functions, with variable intensities and halfwidths, but with central positions defined by d-spacing values calculated from the diffraction crystal planes, corresponding to either spinel or fcc crystal structures by optimizing only the lattice constants of each phase. This procedure allowed localization of the rings indicated in Figure 4, with lattice constants of 8.286 Å for zinc/calcium ferrite and 4.055 Å for silver. The diffraction planes corresponding to the peaks are (1 1 1); (2 2 0); (3 1 1); (4 0 0); (4 2 2); (3 3 3) +(5 1 1); (4 4 0); (6 2 0) and (5 3 3) for zinc/calcium ferrite, and (1 1 1) and (2 2 0) for silver. Additional diffraction spots, not used in the circular profile, can be identified to the (4 4 4) +(7 1 1); (5 5 1); (6 4 2) and (7 3 1) +(5 5 3) diffraction planes of zinc/calcium ferrite and (3 1 1) and (2 2 2) of silver, being marked with *in Figure 4. Materials 2019,12, 3582 9 of 16 Materials 2019, 12, x FOR PEER REVIEW 6 of 17 Figure 3. TEM images of the synthesized nanoparticles. (A,B): Zinc/calcium ferrite nanoparticles; (C): Particle size histogram of image (B) and fitting to a Gaussian distribution; (D): Aspect ratio histogram of particles in image (B); (E,F): Zinc/calcium ferrite nanoparticles containing silver. Small area electron diffraction (SAED) images of zinc/calcium ferrite samples without (Figure 4A) and with silver (Figure 4B) show diffraction spots that can be associated with ferrite phase (cyan rings) and silver (orange rings), as follows. The circular profile of the images was obtained using the radial profile ImageJ plugin and fitted to a sum of Gaussian functions, with variable intensities and halfwidths, but with central positions defined by d-spacing values calculated from the diffraction crystal planes, corresponding to either spinel or fcc crystal structures by optimizing only the lattice constants of each phase. This procedure allowed localization of the rings indicated in Figure 4, with lattice constants of 8.286 Å for zinc/calcium ferrite and 4.055 Å for silver. The diffraction planes corresponding to the peaks are (1 1 1); (2 2 0); (3 1 1); (4 0 0); (4 2 2); (3 3 3) + (5 1 1); (4 4 0); (6 2 0) and (5 3 3) for zinc/calcium ferrite, and (1 1 1) and (2 2 0) for silver. Additional diffraction spots, not used in the circular profile, can be identified to the (4 4 4) + (7 1 1); (5 5 1); (6 4 2) and (7 3 1) + (5 5 3) diffraction planes of zinc/calcium ferrite and (3 1 1) and (2 2 2) of silver, being marked with * in Figure 4. Figure 4. TEM SAED images of the synthesized zinc/calcium ferrite nanoparticles, without (A) and with silver (B). Below each image, a radial profile together with a fit is represented, considering the diffraction lines of zinc/calcium ferrite (marked by cyan triangles) and silver (marked by orange circles). Rings marked by asterisks pass by additional diffraction spots, but did not define a complete circle, not being possible to use in the radial profile. 3.2. Magnetic Properties Figure 4. TEM SAED images of the synthesized zinc/calcium ferrite nanoparticles, without ( A ) and with silver ( B ). Below each image, a radial profile together with a fit is represented, considering the diffraction lines of zinc/calcium ferrite (marked by cyan triangles) and silver (marked by orange circles). Rings marked by asterisks pass by additional diffraction spots, but did not define a complete circle, not being possible to use in the radial profile. 3.2. Magnetic Properties The magnetic properties of the prepared zinc/calcium ferrite nanoparticles, with and without silver coating (Figure 5) were characterized by measuring their magnetic hysteresis loop, which shows the relationship between the induced magnetic moment and the applied magnetic field (H). The calcined nanoparticles present a superparamagnetic behavior, as the ratio between remnant magnetization (M r ) and maximum magnetization (M s ) is below 0.1 (Table 4). If below 0.1, this ratio indicates that more than 90% of the magnetization is lost upon the removal of the applied magnetic field [ 31 , 32 ]. The very low maximum magnetization of the non-calcined nanoparticles is justified by their highly amorphous nature, as verified by XRD. With calcination, the maximum magnetization increases ten times (Figure 5), maintaining a low coercivity. Table 4. Coercive field (H c ), saturation magnetization (M s ), remnant magnetization (M r ) and ratio Mr/Msfor zinc/calcium ferrites at room temperature. - Hc(Oe) Ms(emu/g) Mr(emu/g) Mr/Ms Zn0.5Ca0.5Fe2O4non-calcined 1.8 2.41 8×10−53×10−5 Zn0.5Ca0.5Fe2O4calcined 7.5 20.45 0.022 1×10−3 Silver coated Zn0.5Ca0.5Fe2O4calcined 5.3 18.14 0.016 9×10−4 Materials 2019,12, 3582 16 of 16 26. Solano, E.; Frontera, C.; Puig, T.; Obradors, X.; Ricart, S.; Ros, J. Neutron and X-ray diffraction study of ferrite nanocrystals obtained by microwave-assisted growth. A structural comparison with the thermal synthetic route. J. Appl. Crystallogr. 2014,47, 414–420. [CrossRef] 27. 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